1. Field
This disclosure is generally related to solar cells. More specifically, this disclosure is related to a heterojunction solar cell fabricated by epitaxially depositing a crystalline-Si thin film on a metallurgical grade Si (MG-Si) substrate.
2. Related Art
The negative environmental impact caused by the use of fossil fuels and their rising cost have resulted in a dire need for cleaner, cheaper alternative energy sources. Among different forms of alternative energy sources, solar power has been favored for its cleanness and wide availability.
A solar cell converts light into electricity using the photoelectric effect. There are several basic solar cell structures, including a single p-n junction, p-i-n/n-i-p, and multi-junction. A typical single p-n junction structure includes a p-type doped layer and an n-type doped layer. Solar cells with a single p-n junction can be homojunction solar cells or heterojunction solar cells. If both the p-doped and n-doped layers are made of similar materials (materials with equal band gaps), the solar cell is called a homojunction solar cell. In contrast, a heterojunction solar cell includes at least two layers of materials of different bandgaps. A p-i-n/n-i-p structure includes a p-type doped layer, an n-type doped layer, and an intrinsic (undoped) semiconductor layer (the i-layer) sandwiched between the p-layer and the n-layer. A multi junction structure includes multiple single junction structures of different bandgaps stacked on top of one another.
In a solar cell, light is absorbed near the p-n junction generating carriers. The carriers diffuse into the p-n junction and are separated by the built-in electric field, thus producing an electrical current across the device and external circuitry. An important metric in determining a solar cell's quality is its energy-conversion efficiency, which is defined as the ratio between power converted (from absorbed light to electrical energy) and power collected when the solar cell is connected to an electrical circuit.
For homojunction solar cells, minority-carrier recombination at the cell surface due to the existence of dangling bonds can significantly reduce the solar cell efficiency; thus, a good surface passivation process is needed. In addition, the relatively thick, heavily doped emitter layer, which is formed by dopant diffusion, can drastically reduce the absorption of short wavelength light. Comparatively, heterojunction solar cells, such as Si heterojunction (SHJ) solar cells, are advantageous. FIG. 1 presents a diagram illustrating an exemplary SHJ solar cell (prior art). SHJ solar cell 100 includes front electrodes 102, an n+ amorphous-silicon (n+ a-Si) emitter layer 104, an intrinsic a-Si layer 106, a p-type doped crystalline-Si substrate 108, and an Al backside electrode 110. Arrows in FIG. 1 indicate incident sunlight. Because there is an inherent bandgap offset between a-Si layer 106 and crystalline-Si layer 108, a-Si layer 106 can be used to reduce the surface recombination velocity by creating a barrier for minority carriers. The a-Si layer 106 also passivates the surface of crystalline-Si layer 108 by repairing the existing Si dangling bonds. Moreover, the thickness of n+ a-Si emitter layer 104 can be much thinner compared to that of a homojunction solar cell. Thus, SHJ solar cells can provide a higher efficiency with higher open-circuit voltage (Voc) and larger short-circuit current (Jsc).
Fuhs et al. first reported a hetero-structure based on a-Si and crystalline-Si that generates photocurrent in 1974 (see W. Fuhs et al., “Heterojunctions of amorphous silicon & silicon single crystal”, Int. Conf., Tetrahedrally Bonded Amorphous Semiconductors, Yorktown Hts., N.Y., (1974), pp. 345-350). U.S. Pat. No. 4,496,788 disclosed a heterojunction type solar cell based on stacked a-Si and crystalline-Si wafers. The so-called HIT (heterojunction with intrinsic thin layer) solar cell, which includes an intrinsic a-Si layer interposed between a-Si and crystalline-Si layers, was disclosed by U.S. Pat. No. 5,213,628. However, all these SHJ solar cells are based on a crystalline-Si substrate whose thickness can be between 200 μm and 300 μm. Due to the soaring cost of Si material, the existence of such a thick crystalline-Si substrate significantly increases the manufacture cost of existing SHJ solar cells. To solve the problem of high cost incurred by crystalline-Si wafers, this disclosure presents a solution for SHJ solar cells based on epitaxial mono-crystalline Si thin film grown on low-cost MG-Si wafers.